When considering the interface provided by the GNU C Library (glibc),
thoughts naturally turn to the programming interface as specified by POSIX,
along with numerous extensions added over the years. But glibc also
provides a “tunables” interface to control how the library operates; rather
than being managed by a C API, tunables are set with the
GLIBC_TUNABLES environment
variable. Glibc tunables have been a part of a few security problems
involving setuid binaries, most recently the “Looney
Tunables” bug disclosed at the beginning of October. The glibc
developers are now considering significant changes to tunable handling in
the hope of avoiding such problems in the future.

Source: LWN.net – [$] Toward safer GNU C Library tunable handling